Lithium initiator system

ABSTRACT

This invention reveals an initiator system which is comprised of (a) a lithium initiator, (b) a sodium alkoxide and (c) a polar modifier; wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.05:1 to about 3:1; and wherein the molar ratio of the polar modifier to the lithium initiator is within the range of about 0.25:1 to about 5:1. The subject invention also discloses a process for preparing 3,4-polyisoprene rubber which comprises polymerizing isoprene monomer with a lithium initiator at a temperature which is within the range of about 30° C. to about 100° C. in the presence of a sodium alkoxide and a polar modifier, wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.05:1 to about 3:1; and wherein the molar ratio of the polar modifier to the lithium initiator is within the range of about 0.25:1 to about 5:1.

This is a divisional of U.S. patent application Ser. No. 08/685,762, filed on Jul. 24, 1996, now issued as U.S. Pat. No. 5,677,402, which was a continuation-in-part of U.S. patent application Ser. No. 08/622,272, filed on Mar. 27, 1996, now issued as U.S. Pat. No. 5,654,384, which is a divisional of U.S. patent application Ser. No. 08/531,841, filed on Sep. 22, 1995, now issued as U.S. Pat. No. 5,534,592.

BACKGROUND OF THE INVENTION

It is highly desirable for tires to exhibit good traction characteristics on both dry and wet surfaces. However, it has traditionally been very difficult to improve the traction characteristics of a tire without compromising its rolling resistance and tread wear. Low rolling resistance is important because good fuel economy is virtually always an important consideration. Good tread wear is also an important consideration because it is generally the most important factor which determines the life of the tire.

The traction, tread wear and rolling resistance of a tire is dependent to a large extent on the dynamic viscoelastic properties of the elastomers utilized in making the tire tread. In order to reduce the rolling resistance of a tire, rubbers having a high rebound have traditionally been utilized in making the tire's tread. On the other hand, in order to increase the wet skid resistance of a tire, rubbers which undergo a large energy loss have generally been utilized in the tire's tread. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubber material for automobile tire treads. However, such blends are not totally satisfactory for all purposes.

The inclusion of styrene-butadiene rubber (SBR) in tire tread formulations can significantly improve the traction characteristics of tires made therewith. However, styrene is a relatively expensive monomer and the inclusion of SBR in tire tread formulations leads to increased costs.

Carbon black is generally included in rubber compositions which are employed in making tires and most other rubber articles. It is desirable to attain the best possible dispersion of the carbon black throughout the rubber to attain optimized properties. It is also highly desirable to improve the interaction between the carbon black and the rubber. By improving the affinity of the rubber compound to the carbon black, physical properties can be improved. Silica can also be included in tire tread formulations to improve rolling resistance.

U.S. Pat. No. 4,843,120 discloses that tires having improved performance characteristics can be prepared by utilizing rubbery polymers having multiple glass transition temperatures as the tread rubber. These rubbery polymers having multiple glass transition temperatures exhibit a first glass transition temperature which is within the range of about -110° C. to -20° C. and exhibit a second glass transition temperature which is within the range of about -50° C. to 0° C. According to U.S. Pat. No. 4,843,120, these polymers are made by polymerizing at least one conjugated diolefin monomer in a first reaction zone at a temperature and under conditions sufficient to produce a first polymeric segment having a glass transition temperature which is between -110°0 C. and -20° C. and subsequently continuing said polymerization in a second reaction zone at a temperature and under conditions sufficient to produce a second polymeric segment having a glass transition temperature which is between -20° C. and 20° C. Such polymerizations are normally catalyzed with an organolithium catalyst and are normally carried out in an inert organic solvent.

U.S. Pat. No. 5,137,998 discloses a process for preparing a rubbery terpolymer of styrene, isoprene and butadiene having multiple glass transition temperatures and having an excellent combination of properties for use in making tire treads which comprises terpolymerizing styrene, isoprene and 1,3-butadiene in an organic solvent at a temperature of no more than about 40° C. in the presence of (a) at least one member selected from the group consisting of tripiperidino phosphine oxide and alkali metal alkoxides and (b) an organolithium compound.

U.S. Pat. No. 5,047,483 discloses a pneumatic tire having an outer circumferential tread where said tread is a sulfur-cured rubber composition comprised of, based on 100 parts by weight rubber (phr), (A) about 10 to about 90 parts by weight of a styrene, isoprene and butadiene terpolymer rubber (SIBR), and (B) about 70 to about 30 weight percent of at least one of cis 1,4-polyisoprene rubber and cis 1,4-polybutadiene rubber wherein said SIBR rubber is comprised of (1) about 10 to about 35 weight percent bound styrene, (2) about 30 to about 50 weight percent bound isoprene and (3) about 30 to about 40 weight percent bound butadiene and is characterized by having a single glass transition temperature (Tg) which is in the range of about -10° C. to about -40° C. and, further, the said bound butadiene structure contains about 30 to about 40 percent 1,2-vinyl units, the said bound isoprene structure contains about 10 to about 30 percent 3,4-units and the sum of the percent 1,2-vinyl units of the bound butadiene and the percent 3,4-units of the bound isoprene is in the range of about 40 to about 70 percent.

U.S. Pat. No. 5,272,220 discloses a styrene-isoprene-butadiene rubber which is particularly valuable for use in making truck tire treads which exhibit improved rolling resistance and tread wear characteristics, said rubber being comprised of repeat units which are derived from about 5 weight percent to about 20 weight percent styrene, from about 7 weight percent to about 35 weight percent isoprene and from about 55 weight percent to about 88 weight percent 1,3-butadiene, wherein the repeat units derived from styrene, isoprene and 1,3-butadiene are in essentially random order, wherein from about 25 percent to about 40 percent of the repeat units derived from the 1,3-butadiene are of the cis-microstructure, wherein from about 40 percent to about 60 percent of the repeat units derived from the 1,3-butadiene are of the trans-microstructure, wherein from about 5 percent to about 25 percent of the repeat units derived from the 1,3-butadiene are of the vinyl-microstructure, wherein from about 75 percent to about 90 percent of the repeat units derived from the isoprene are of the 1,4-microstructure, wherein from about 10 percent to about 25 percent of the repeat units derived from the isoprene are of the 3,4-microstructure, wherein the rubber has a glass transition temperature which is within the range of about -90° C. to about -70° C., wherein the rubber has a number average molecular weight which is within the range of about 150,000 to about 400,000, wherein the rubber has a weight average molecular weight of about 300,000 to about 800,000, and wherein the rubber has an inhomogeneity which is within the range of about 0.5 to about 1.5.

U.S. Pat. No. 5,239,009 reveals a process for preparing a rubbery polymer which comprises: (a) polymerizing a conjugated diene monomer with a lithium initiator in the substantial absence of polar modifiers at a temperature which is within the range of about 5° C. to about 100° C. to produce a living polydiene segment having a number average molecular weight which is within the range of about 25,000 to about 350,000; and (b) utilizing the living polydiene segment to initiate the terpolymerization of 1,3-butadiene, isoprene and styrene, wherein the terpolymerization is conducted in the presence of at least one polar modifier at a temperature which is within the range of about 5° C. to about 70° C. to produce a final segment which is comprised of repeat units which are derived from 1,3-butadiene, isoprene and styrene, wherein the final segment has a number average molecular weight which is within the range of about 25,000 to about 350,000. The rubbery polymer made by this process is reported to be useful for improving the wet skid resistance and traction characteristics of tires without sacrificing tread wear or rolling resistance.

U.S. Pat. No. 5,061,765 discloses isoprene-butadiene copolymers having high vinyl contents which can reportedly be employed in building tires which have improved traction, rolling resistance and abrasion resistance. These high vinyl isoprene-butadiene rubbers are synthesized by copolymerizing 1,3-butadiene monomer and isoprene monomer in an organic solvent at a temperature which is within the range of about -10° C. to about 100° C. in the presence of a catalyst system which is comprised of (a) an organoiron compound, (b) an organoaluminum compound, (c) a chelating aromatic amine and (d) a protonic compound; wherein the molar ratio of the chelating amine to the organoiron compound is within the range of about 0.1:1 to about 1:1, wherein the molar ratio of the organoaluminum compound to the organoiron compound is within the range of about 5:1 to about 200:1, and herein the molar ratio of the protonic compound to the organoaluminum compound is within the range of about 0.001:1 to about 0.2:1.

U.S. Pat. No. 5,405,927 discloses an isoprene-butadiene rubber which is particularly valuable for use in making truck tire treads, said rubber being comprised of repeat units which are derived from about 20 weight percent to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent 1,3-butadiene, wherein the repeat units derived from isoprene and 1,3-butadiene are in essentially random order, wherein from about 3 percent to about 10 percent of the repeat units in said rubber are 1,2-polybutadiene units, wherein from about 50 percent to about 70 percent of the repeat units in said rubber are 1,4-polybutadiene units, wherein from about 1 percent to about 4 percent of the repeat units in said rubber are 3,4-polyisoprene units, wherein from about 25 percent to about 40 percent of the repeat units in the polymer are 1,4-polyisoprene units, wherein the rubber has a glass transition temperature which is within the range of about -90° C. to about -75° C., and wherein the rubber has a Mooney viscosity which is within the range of about 55 to about 140.

SUMMARY IF THE INVENTION

The high performance polybutadiene rubber blends of this invention have excellent properties for incorporation into the rubber compounds used in making tire treads. The polybutadiene blends of this invention can be employed to improve the wet skid resistance and traction characteristics of tires without sacrificing tread wear or rolling resistance. A highly desirable combination of tire characteristics can be attained without utilizing styrene-butadiene rubber.

The present invention more specifically discloses a high performance polybutadiene rubber blend for utilization in manufacturing tire tread compounds which is comprised of (a) a first polybutadiene rubber, (b) a second polybutadiene rubber and (c) a third polybutadiene rubber; wherein the first polybutadiene rubber, the second polybutadiene rubber and the third polybutadiene rubber are selected from the group of consisting of super-high vinyl polybutadiene rubbers having a vinyl content which is within the range of 80 percent to 100 percent, high vinyl polybutadiene rubbers having a vinyl content which is within the range of 60 percent to 79 percent, medium vinyl polybutadiene rubbers having a vinyl content which is within the range of 35 percent to 59 percent and low vinyl polybutadiene rubbers having a vinyl content which is within the range of 8 percent to 34 percent; wherein at least three members selected from the group consisting of super-high vinyl polybutadiene rubbers, high vinyl polybutadiene rubbers, medium vinyl polybutadiene rubbers and low vinyl polybutadiene rubbers are included in the blend, wherein at least one of the polybutadiene rubbers in the blend has a vinyl content which is within 35 percentage points of the vinyl content of at least one other polybutadiene rubber in the blend, wherein the vinyl content of the first polybutadiene rubber differs from the vinyl content of the second polybutadiene rubber and the third polybutadiene rubber by at least 5 percentage points, wherein the vinyl content of the second polybutadiene rubber differs from the vinyl content of the third polybutadiene rubber by at least 5 percentage points, wherein the blend as a whole has a total vinyl content of at least 40 percent, and wherein the blend contains at least 10 parts of the first polybutadiene rubber, at least 10 parts of the second polybutadiene rubber and at least 10 parts of the third polybutadiene rubber, based upon a total of 100 parts of rubber in the blend.

The various polybutadiene rubbers employed in the blends of this invention can be made utilizing the techniques of this invention to attain various desired vinyl contents. The subject invention further discloses a process for preparing high vinyl polybutadiene rubber which comprises polymerizing 1,3-butadiene monomer with a lithium initiator at a temperature which is within the range of about 5° C. to about 100° C. in the presence of a sodium alkoxide and a polar modifier, wherein the molar ratio of the sodium alkoxide to the polar modifier is within the range of about 0.1:1 to about 10:1; and wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.05:1 to about 10:1.

The subject invention also specifically discloses an initiator system which is comprised of (a) a lithium initiator, (b) a sodium alkoxide and (c) a polar modifier; wherein the molar ratio of the sodium alkoxide to the polar modifier is within the range of about 0.1:1 to about 10:1; and wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.01:1 to about 20:1.

The present invention further reveals an initiator system which is comprised of (a) a lithium initiator, (b) a sodium alkoxide and (c) a polar modifier; wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.05:1 to about 3:1; and wherein the molar ratio of the polar modifier to the lithium initiator is within the range of about 0.25:1 to about 5:1.

The subject invention also discloses a process for preparing 3,4-polyisoprene rubber which comprises polymerizing isoprene monomer with a lithium initiator at a temperature which is within the range of about 30° C. to about 100° C. in the presence of a sodium alkoxide and a polar modifier, wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.05:1 to about 3:1; and wherein the molar ratio of the polar modifier to the lithium initiator is within the range of about 0.25:1 to about 5:1.

DETAILED DESCRIPTION OF THE INVENTION

The super-high vinyl polybutadiene rubbers, high vinyl polybutadiene rubbers, medium vinyl polybutadiene rubbers and low vinyl polybutadiene rubbers which can be employed in the blends of this invention are typically synthesized by anionic polymerization in an organic medium. The polymerization is normally carried out in an inert organic medium utilizing a lithium catalyst. The vinyl content of the polybutadiene rubber made is controlled by the amount of modifier system present during the polymerization step.

The inert organic medium which is utilized as the solvent will typically be a hydrocarbon which is liquid at ambient temperatures which can be one or more aromatic, paraffinic or cycloparaffinic compounds. These solvents will normally contain from 4 to 10 carbon atoms per molecule and will be liquids under the conditions of the polymerization. It is, of course, important for the solvent selected to be inert. The term "inert" as used herein means that the solvent does not interfere with the polymerization reaction or react with the polymers made thereby. Some representative examples of suitable organic solvents include pentane, isooctane, cyclohexane, normal hexane, benzene, toluene, xylene, ethylbenzene and the like, alone or in admixture. Saturated aliphatic solvents, such as cyclohexane and normal hexane, are most preferred.

The lithium catalysts which can be used are typically organolithium compounds. The organolithium compounds which are preferred can be represented by the formula R-Li, wherein R represents a hydrocarbyl radical containing from 1 to about 20 carbon atoms. Generally, such monofunctional organolithium compounds will contain from 1 to about 10 carbon atoms. Some representative examples of organolithium compounds which can be employed include methyllithium, ethyllithium, isopropyllithium, n-butyllithium, sec-butyllithium, n-octyllithium, tert-octyllithium, n-decyllithium, phenyllithium, 1-napthyllithium, 4-butylphenyllithium, p-tolyllithium, 1-naphthyllithium, 4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium, cyclohexyllithium, 4-butylcyclohexyllithium and 4-cyclohexylbutyllithium. organo monolithium compounds, such as alkyllithium compounds and aryllithium compounds, are usually employed. Some representative examples of preferred organo monolithium compounds that can be utilized include ethyllithium, isopropyllithium, n-butyllithium, secondary-butyllithium, normal-hexyllithium, tertiary-octyllithium, phenyllithium, 2-napthyllithium, 4-butylphenyllithium, 4-phenylbutyllithium, cyclohexyllithium and the like. Normal-butyllithium and secondary-butyllithium are highly preferred lithium initiators.

The amount of lithium catalyst utilized will vary from one organolithium compound to another and with the molecular weight that is desired for the polybutadiene rubber being synthesized. As a general rule in all anionic polymerizations, the molecular weight (Mooney viscosity) of the polymer produced is inversely proportional to the amount of catalyst utilized. As a general rule, from about 0.01 phm (parts per hundred parts by weight of monomer) to 1 phm of the lithium catalyst will be employed. In most cases, from 0.01 phm to 0.1 phm of the lithium catalyst will be employed with it being preferred to utilize 0.025 phm to 0.07 phm of the lithium catalyst.

Normally, from about 5 weight percent to about 35 weight percent of the 1,3-butadiene monomer will be charged into the polymerization medium (based upon the total weight of the polymerization medium including the organic solvent and monomer). In most cases, it will be preferred for the polymerization medium to contain from about 10 weight percent to about 30 weight percent monomer. It is typically more preferred for the polymerization medium to contain from about 20 weight percent to about 25 weight percent monomer.

The 1,3-butadiene will be polymerized at a temperature which is within the range of about 5° C. to about 100° C. The polymerization temperature will preferably be within the range of about 40° C. to about 90° C. for practical reasons and to attain the desired microstructure. Temperatures within the range of about 60° C. to about 80° C. are most preferred. The microstructure of the polybutadiene rubber being prepared is somewhat dependent upon the polymerization temperature.

The polymerization is allowed to continue until essentially all of the 1,3-butadiene monomer has been exhausted. In other words, the polymerization is allowed to run to completion. Since a lithium catalyst is employed to polymerize the 1,3-butadiene monomer, a living polybutadiene rubber is produced. The living polybutadiene rubber synthesized will have a number average molecular weight which is within the range of about 25,000 to about 700,000. The polybutadiene rubber synthesized will more typically have a number average molecular weight which is within the range of about 150,000 to about 400,000.

To increase the level of vinyl content, the polymerization can be carried out in the presence of at least one polar modifier. Ethers and tertiary amines which act as Lewis bases are representative examples of polar modifiers that can be utilized. Some specific examples of typical polar modifiers include diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine, triethylamine, N,N,N',N'-tetramethylethylenediamine, N-methyl morpholine, N-ethyl morpholine, N-phenyl morpholine and the like.

The modifier can also be a 1,2,3-trialkoxybenzene or a 1,2,4-trialkoxybenzene. Some representative examples of 1,2,3-trialkoxybenzenes that can be used include 1,2,3-trimethoxybenzene, 1,2,3-triethoxybenzene, 1,2,3-tributoxybenzene, 1,2,3-trihexoxybenzene, 4,5,6-trimethyl-1,2,3-trimethoxybenzene, 4,5,6-tri-n-pentyl-1,2,3-triethoxybenzene, 5-methyl-1,2,3-trimethoxybenzene and 5-propyl-1,2,3-trimethoxybenzene. Some representative examples of 1,2,4-trialkoxybenzenes that can be used include 1,2,4-trimethoxybenzene, 1,2,4-triethoxybenzene, 1,2,4-tributoxybenzene, 1,2,4-tripentoxybenzene, 3,5,6-trimethyl-1,2,4-trimethoxybenzene, 5-propyl-1,2,4-trimethoxybenzene and 3,5-dimethyl-1,2,4-trimethoxybenzene. Dipiperidinoethane, dipyrrolidinoethane, tetramethylethylene diamine, diethylene glycol, dimethyl ether and tetrahydrofuran are representative of highly preferred modifiers. U.S. Pat. No. 4,022,959 describes the use of ethers and tertiary amines as polar modifiers in greater detail.

The utilization of 1,2,3-trialkoxybenzenes and 1,2,4-trialkoxybenzenes as modifiers is described in greater detail in U.S. Pat. No. 4,696,986. The teachings of U.S. Pat. No. 4,022,959 and U.S. Pat. No. 4,696,986 are incorporated herein by reference in their entirety. The microstructure of the repeat units which are derived from butadiene monomer is a function of the polymerization temperature and the amount of polar modifier present. For example, it is known that higher temperatures result in lower vinyl contents (lower levels of 1,2-microstructure). Accordingly, the polymerization temperature, quantity of modifier and specific modifier selected will be determined with the ultimate desired microstructure of the polybutadiene rubber being synthesized being kept in mind.

In cases where super-high vinyl polybutadiene rubber is being synthesized, a combination of a sodium alkoxide and polar modifier will normally be employed as the modifier system. It has been unexpectedly found that a combination of a sodium alkoxide and polar modifier acts synergistically to increase the vinyl content of polybutadiene rubber synthesized in its presence. The utilization of this synergistic modifier system can also be employed advantageously in the synthesis of high vinyl polybutadiene rubber.

The sodium alkoxides which can be utilized in the catalyst system will normally be of the formula NaOR, wherein R is an alkyl group containing from about 2 to about 12 carbon atoms. The sodium metal alkoxide will typically contain from about 2 to about 12 carbon atoms. It is generally preferred for the sodium alkoxide to contain from about 3 to about 8 carbon atoms. It is generally most preferred for the sodium alkoxide to contain from about 4 to about 6 carbon atoms. Sodium t-amyloxide (sodium t-pentoxide) is a representative example of a preferred sodium alkoxides which can be utilized in the modifier systems of this invention.

The molar ratio of the sodium alkoxide to the polar modifier in the modifier system will normally be within the range of about 0.1:1 to about 10:1 and the molar ratio of the sodium alkoxide to the lithium initiator will normally be within the range of about 0.01:1 to about 20:1. It is generally preferred for the molar ratio of the sodium alkoxide to the polar modifier to be within the range of about 0.2:1 to about 5:1 and for the molar ratio of the sodium alkoxide to the lithium initiator to be within the range of about 0.05:1 to about 10:1. It is generally more preferred for the molar ratio of the sodium alkoxide to the polar modifier to be within the range of about 0.5:1 to about 1:1 and for the molar ratio of the sodium alkoxide to the lithium initiator to be within the range of about 0.2:1 to about 3:1.

After the polymerization has been completed, the polybutadiene rubber can be recovered from the organic solvent. The polybutadiene rubber can be recovered from the organic solvent and residue by any means, such as decantation, filtration, centrification and the like. It is often desirable to precipitate the polybutadiene rubber from the organic solvent by the addition of lower alcohols containing from about 1 to about 4 carbon atoms to the polymer solution. Suitable lower alcohols for precipitation of the polybutadiene rubber from the polymer cement include methanol, ethanol, isopropyl alcohol, normal-propyl alcohol and t-butyl alcohol. The utilization of lower alcohols to precipitate the polybutadiene rubber from the polymer cement also "kills" the living polymer by inactivating lithium end groups. After the polybutadiene rubber is recovered from the solution, steam-stripping can be employed to reduce the level of volatile organic compounds in the polymer.

It has been determined that the lithium initiator systems of this invention can also be used in the polymerization of isoprene monomer and the copolymerization of 1,3-butadiene and vinyl aromatic monomers. For instance, isoprene monomer can be polymerized into 3,4-polyisoprene and butadiene and styrene can be copolymerized into styrene-butadiene rubber (SBR) using the lithium initiator systems of this invention. These initiator systems are comprised of (a) a lithium initiator, (b) a sodium alkoxide and (c) a polar modifier.

The combination of sodium alkoxide and polar modifier used in the initiator system of this invention causes polyisoprene rubber produced therewith to have a very high 3,4-isomer content and greatly improves (increases) the rate of polymerization. It has been unexpectedly found that the combination of a sodium alkoxide and a polar modifier acts synergistically to increase the 3,4-isomer content of polyisoprene rubber synthesized in its presence.

Since it will normally be desirable for the 3,4-polyisoprene synthesized with the lithium initiator systems of this invention to have a relatively low molecular weight, the amount of lithium initiator employed will be about three times as much as is used in synthesizing conventional 3,4-polyisoprene rubber. As a general rule, from about 0.02 phm (parts per hundred parts by weight of monomer) to about 1 phm of the lithium catalyst will be employed. In most cases, from about 0.03 phm to about 0.3 phm of the lithium catalyst will be employed in the synthesis of 3,4-polyisoprene rubber. It is typically most preferred to utilize from 0.06 phm to 0.2 phm of the lithium catalyst in the synthesis of 3,4-polyisoprene rubber.

Normally, from about 5 weight percent to about 35 weight percent of the isoprene monomer will be charged into the polymerization medium (based upon the total weight of the polymerization medium including the organic solvent and monomer). In most cases, it will be preferred for the polymerization medium to contain from about 10 weight percent to about 30 weight percent monomer. It is typically more preferred for the polymerization medium to contain from about 20 weight percent to about 25 weight percent monomer.

The isoprene will be polymerized at a temperature which is within the range of about 30° C. to about 100° C. The polymerization temperature will preferably be within the range of about 40° C. to about 70° C. for practical reasons and to attain the desired microstructure. Temperatures within the range of about 50° C. to about 60° C. are most preferred. The microstructure of the 3,4-polyisoprene rubber being prepared is somewhat dependent upon the polymerization temperature.

The microstructure of the repeat units which are derived from isoprene monomer is a function of the polymerization temperature and the amount of modifier present. For example, higher temperatures result in lower 3,4-isomer contents (lower levels of 3,4-microstructure). Accordingly, the polymerization temperature, quantity of modifier and specific modifier selected will be determined with the ultimate desired microstructure of the polyisoprene rubber being synthesized being kept in mind.

The polymerization is allowed to continue until essentially all of the isoprene monomer has been exhausted. In other words, the polymerization is allowed to run to completion. Since a lithium catalyst is employed to polymerize the isoprene monomer, a living 3,4-polyisoprene rubber is produced.

The living polyisoprene rubber synthesized will typically have a number average molecular weight which is within the range of about 30,000 to about 180,000 and a weight average molecular weight which is within the range of about 40,000 to about 300,000. The 3,4-polyisoprene rubber synthesized will more typically have a number average molecular weight which is within the range of about 50,000 to about 150,000 and will preferably have a number average molecular weight which is within the range of about 70,000 to about 120,000.

The 3,4-polyisoprene rubber will typically have a high glass transition temperature (Tg) which is within the range of about 0° C. to about 25° C. It will more typically have a glass transition temperature which is within the range of about 5° C. to about 20° C. The 3,4-polyisoprene rubber will generally have a 3,4-isomer content which is within the range of about 75 percent to about 95 percent and a 1,2-isomer content which is within the range of about 5 percent to about 25 percent.

In the synthesis of 3,4-polyisoprene rubber, the molar ratio of the sodium alkoxide to the lithium initiator will normally be within the range of about 0.05:1 to about 3:1 and the molar ratio of the polar modifier to the lithium initiator will normally be within the range of about 0.25:1 to about 5:1. It is generally preferred for the molar ratio of the sodium alkoxide to the lithium initiator to be within the range of about 0.15:1 to about 1:1 and for the molar ratio of the polar modifier to the lithium initiator to be within the range of about 0.4:1 to about 3:1. It is generally more preferred for the molar ratio of the sodium alkoxide to the lithium initiator to be within the range of about 0.3:1 to about 0.5:1 and for the molar ratio of the polar modifier to the lithium initiator to be within the range of about 0.5:1 to about 2:1.

There are valuable benefits associated with utilizing the specific polybutadiene rubber blend of this invention in making tire tread compounds. Tire tread compounds can be made using only the polybutadiene rubbers of this invention without the need to blend additional rubbers, such as styrene-butadiene rubber, therein. There are also valuable benefits associated with utilizing the 3,4-polyisoprene rubber which is made with the lithium initiator systems of this invention in making tire tread compounds. For instance, traction characteristics can be significantly improved without compromising tread wear or rolling resistance. As a general rule, from about 5 phr (parts per 100 parts of rubber) to about 50 phr of the high Tg 3,4-polyisoprene will be included in tire tread compounds. Normally, such tire tread compounds will contain from about 10 phr to 25 phr of the high Tg 3,4-polyisoprene. It is typically more preferred for tire tread compounds to contain from about 12 phr to about 20 phr of the high Tg 3,4-polyisoprene rubber. Such tire tread compounds will, of course, also contain at least one other rubber which is co-curable with the 3,4-polyisoprene. Some representative examples of other rubbers which are co-curable with the 3,4-polyisoprene rubber include natural rubber, high cis-1,4-polybutadiene rubber, high vinyl polybutadiene rubber, medium vinyl polybutadiene rubber, high trans-1,4-polybutadiene rubber, solution styrene-butadiene rubber, emulsion styrene-butadiene rubber, styrene-isoprene-butadiene rubber, styrene-isoprene rubber, isoprene-butadiene rubber and other types of 3,4-polyisoprene rubber. These benefits are explained more fully in U.S. patent application Ser. No. 08/642,180, filed on May 6, 1996.

The high performance polybutadiene rubber blends of this invention contain at least three different polybutadiene rubbers having different vinyl contents. The blend can include: (1) super-high vinyl polybutadiene rubber which has a vinyl content which is within the range of 80 percent to 100 percent and a glass transition temperature which is within the range of about -15° C. to about 0° C., (2) high vinyl polybutadiene rubber which has a vinyl content which is within the range of 60 percent to 79 percent and a glass transition temperature which is within the range of about -45° C. to about -40° C., (3) medium vinyl polybutadiene rubber which has a vinyl content which is within the range of 35 percent to 59 percent and a glass transition temperature which is within the range of about -75° C. to about -45° C. and (4) low vinyl polybutadiene rubber which has a vinyl content which is within the range of 8 percent to 34 percent and a glass transition temperature which is within the range of about -95° C. to about -75° C. However, it is critical for the blend to contain at least three of the four members of the group consisting of super-high vinyl polybutadiene rubber, high vinyl polybutadiene rubber, medium vinyl polybutadiene rubber and low vinyl polybutadiene rubber.

It is also critical for at least one of the polybutadiene rubbers in the blend to have a vinyl content which is within 35 percentage points of the vinyl content of at least one other polybutadiene rubber in the blend to provide compatibility. It is preferred for at least one of the polybutadiene rubbers in the blend to have a vinyl content which is within 30 percentage points of the vinyl content of at least one other polybutadiene rubber in the blend. It is more preferred for at least one of the polybutadiene rubbers in the blend to have a vinyl content which is within 25 percentage points of the vinyl content of at least one other polybutadiene rubber in the blend. For instance, it would be highly preferred for the blend to contain a super-high vinyl polybutadiene rubber having a vinyl content of 90 percent and a high vinyl polybutadiene rubber having a vinyl content of 65 percent (the vinyl content of the high vinyl polybutadiene rubber differs from the vinyl content of the super-high vinyl polybutadiene by only 25 percentage points).

It is also important for the three different polybutadiene rubbers employed in the blend to have vinyl contents which differ from the other two polybutadiene rubbers employed in the blend by at least 5 percentage points. In other words, the vinyl contents of the different polybutadiene rubbers utilized in the blend must differ by at least 5 percentage points. For example, if a super-high vinyl polybutadiene rubber having a vinyl content of 80 percent and a high vinyl polybutadiene rubber are employed in the blend, the vinyl content of the high vinyl polybutadiene must be less than 75 percent. It is preferred for the three different polybutadiene rubbers employed in the blend to have vinyl contents which differ from the other two polybutadiene rubbers employed in the blend by at least 10 percentage points. Thus, it would be highly preferred to utilize a super-high vinyl polybutadiene rubber having a vinyl content of 85 percent and a high vinyl polybutadiene rubber having a vinyl content of 70 percent in the blend (there is a 15 percentage point difference between the vinyl contents of the two polybutadiene rubbers). Stated in still another way, the vinyl content of the first polybutadiene rubber cannot have a vinyl content which is within 5 percentage points of the vinyl content of the second polybutadiene rubber or the third polybutadiene rubber, and the vinyl content of the second polybutadiene rubber cannot have a vinyl content which is within 5 percentage points of the vinyl content of the third polybutadiene rubber.

It is also important for the blend as a whole to have a total vinyl content of at least 40 percent and preferably 45 percent. The total vinyl content of the blend as a whole is the sum of the products of the number of parts of each of the polybutadiene rubbers included in the blend and the vinyl contents of those polybutadiene rubbers, with that sum being divided by the total number of parts of polybutadiene rubber included in the blend. For example, if the blend included 40 parts of a low vinyl polybutadiene rubber having a vinyl content of 20 percent, 40 parts of a medium vinyl polybutadiene rubber having a vinyl content of 40 percent and 20 parts of a super-high vinyl polybutadiene rubber having a vinyl content of 80 percent, the blend as a whole would have a total vinyl content of 40 percent. In another example, if the blend included 20 parts of a low vinyl polybutadiene rubber having a vinyl content of 30 percent, 40 parts of a high vinyl polybutadiene rubber having a vinyl content of 60 percent and 40 parts of a super-high vinyl polybutadiene rubber having a vinyl content of 90 percent, the blend as a whole would have a total vinyl content of 66 percent.

The blends of this invention will contain at least 10 phr (parts per 100 parts by weight of rubber) of the first polybutadiene rubber, at least 10 phr of the second polybutadiene rubber and at least 10 phr of the third polybutadiene rubber. The blends of this invention will preferably contain at least 20 phr of the first polybutadiene rubber, at least 20 phr of the second polybutadiene rubber and at least 20 phr of the third polybutadiene rubber. The blends of this invention will more preferably contain at least 25 phr of the first polybutadiene rubber, at least 25 phr of the second polybutadiene rubber and at least 25 phr of the third polybutadiene rubber.

The polybutadiene rubber blends of this invention can be compounded utilizing conventional ingredients and standard techniques. For instance, the polybutadiene rubber blends will typically be mixed with carbon black and/or silica, sulfur, fillers, accelerators, oils, waxes, scorch inhibiting agents and processing aids. In most cases, the polybutadiene rubber blends will be compounded with sulfur and/or a sulfur-containing compound, at least one filler, at least one accelerator, at least one antidegradant, at least one processing oil, zinc oxide, optionally a tackifier resin, optionally a reinforcing resin, optionally one or more fatty acids, optionally a peptizer and optionally one or more scorch inhibiting agents. Such blends will normally contain from about 0.5 to 5 phr (parts per hundred parts of rubber by weight) of sulfur and/or a sulfur-containing compound with 1 phr to 2.5 phr being preferred. It may be desirable to utilize insoluble sulfur in cases where bloom is a problem.

Normally from 10 to 150 phr of at least one filler will be utilized in the blend with 30 to 80 phr being preferred. In most cases, at least some carbon black will be utilized in the filler. The filler can, of course, be comprised totally of carbon black. Silica can be included in the filler to improve tear resistance and heat buildup. Clays and/or talc can be included in the filler to reduce cost. The blend will also normally include from 0.1 to 2.5 phr of at least one accelerator with 0.2 to 1.5 phr being preferred. Antidegradants, such as antioxidants and antiozonants, will generally be included in the tread compound blend in amounts ranging from 0.25 to 10 phr with amounts in the range of 1 to 5 phr being preferred. Processing oils will generally be included in the blend in amounts ranging from 2 to 100 phr with amounts ranging from 5 to 50 phr being preferred. The polybutadiene blends of this invention will also normally contain from 0.5 to 10 phr of zinc oxide with 1 to 5 phr being preferred. These blends can optionally contain from 0 to 10 phr of tackifier resins, 0 to 10 phr of reinforcing resins, 1 to 10 phr of fatty acids, 0 to 2.5 phr of peptizers and 0 to 1 phr of scorch inhibiting agents.

To fully realize the total advantages of the blends of this invention, silica will normally be included in the tread rubber formulation. The processing of the polybutadiene rubber blend is normally conducted in the presence of a sulfur containing organosilicon compound to realize maximum benefits. Examples of suitable sulfur-containing organosilicon compounds are of the formula:

    Z-Alk-S.sub.n -Alk-Z                                       (I)

in which Z is selected from the group consisting of ##STR1## where R¹ is an alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl; wherein R² is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbon atoms; and wherein Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and n is an integer of 2 to 8.

Specific examples of sulfur-containing organosilicon compounds which may be used in accordance with the present invention include: 3,3'-bis(trimethoxysilylpropyl) disulfide, 3,3'-bis(triethoxysilylpropyl) tetrasulfide, 3,3'-bis(triethoxysilylpropyl) octasulfide, 3,3'-bis(trimethoxysilylpropyl) tetrasulfide, 2,2'-bis(triethoxysilylethyl) tetrasulfide, 3,3'-bis(trimethoxysilylpropyl) trisulfide, 3,3'-bis(triethoxysilylpropyl) trisulfide, 3,3'-bis(tributoxysilylpropyl) disulfide, 3,3+-bis(trimethoxysilylpropyl) hexasulfide, 3,3'-bis(trimethoxysilylpropyl) octasulfide, 3,3'-bis(trioctoxysilylpropyl) tetrasulfide, 3,3'-bis(trihexoxysilylpropyl) disulfide, 3,3'-bis(tri-2"- ethylhexoxysilylpropyl) trisulfide, 3,3'-bis(triisooctoxysilylpropyl) tetrasulfide, 3,3'-bis(tri-t-butoxysilylpropyl) disulfide, 2,2'-bis(methoxy diethoxy silyl ethyl) tetrasulfide, 2,2'-bis(tripropoxysilylethyl) pentasulfide, 3,3'-bis(tricyclonexoxysilylpropyl) tetrasulfide, 3,3'-bis(tricyclopentoxysilylpropyl) trisulfide, 2,2'-bis(tri-2"-methylcyclohexoxysilylethyl) tetrasulfide, bis(trimethoxysilylmethyl) tetrasulfide, 3-methoxy ethoxy propoxysilyl 3'-diethoxybutoxysilylpropyltetrasulfide, 2,2'-bis(dimethyl methoxysilylethyl) disulfide, 2,2'-bis(dimethyl sec.butoxysilylethyl) trisulfide, 3,3'-bis(methyl butylethoxysilylpropyl) tetrasulfide, 3,3'-bis(di t-butylmethoxysilylpropyl) tetrasulfide, 2,2'-bis(phenyl methyl methoxysilylethyl) trisulfide, 3,3'-bis(diphenyl isopropoxysilyipropyl) tetrasulfide, 3,3'-bis(diphenyl cyclohexoxysilylpropyl) disulfide, 3,3'-bis(dimethyl ethylmercaptosilylpropyl) tetrasulfide, 2,2'-bis(methyl dimethoxysilylethyl) trisulfide, 2,2'-bis(methyl ethoxypropoxysilylethyl) tetrasulfide, 3,3'-bis(diethyl methoxysilylpropyl) tetrasulfide, 3,3'-bis(ethyl di-sec. butoxysilylpropyl) disulfide, 3,3'-bis(propyl diethoxysilylpropyl) disulfide, 3,3'-bis(butyl dimethoxysilylpropyl) trisulfide, 3,3'-bis(phenyl dimethoxysilylpropyl) tetrasulfide, 3-phenyl ethoxybutoxysilyl 3'-trimethoxysilylpropyl tetrasulfide, 4,4'-bis(trimethoxysilylbutyl) tetrasulfide, 6,6'-bis(triethoxysilylhexyl) tetrasulfide, 12,12'-bis(triisopropoxysilyl dodecyl) disulfide, 18,18'-bis(trimethoxysilyloctadecyl) tetrasulfide, 18,18'-bis(tripropoxysilyloctadecenyl) tetrasulfide, 4,4'-bis(trimethoxysilyl-buten-2-yl) tetrasulfide, 4,4'-bis(trimethoxysilylcyclohexylene) tetrasulfide, 5,5'-bis(dimethoxymethylsilylpentyl) trisulfide, 3,3'-bis(trimethoxysilyl-2-methylpropyl) tetrasulfide, 3,3'-bis(dimethoxyphenylsilyl-2-methylpropyl) disulfide.

The preferred sulfur-containing organosilicon compounds are the 3,3'-bis(trimethoxy or triethoxy silylpropyl) sulfides. The most preferred compound is 3,3'-bis(triethoxysilylpropyl) tetrasulfide.

Therefore as to formula I, preferably Z is ##STR2## where R² is an alkoxy of 2 to 4 carbon atoms, with 2 carbon atoms being particularly preferred; Alk is a divalent hydrocarbon of 2 to 4 carbon atoms with 3 carbon atoms being particularly preferred; and n is an integer of from 3 to 5 with 4 being particularly preferred.

The amount of the sulfur-containing organosilicon compound of formula I in a rubber composition will vary, depending on the level of silica that is used. Generally speaking, the amount of the compound of formula I will range from about 0.01 to about 1.0 parts by weight per part by weight of the silica. Preferably, the amount will range from about 0.02 to about 0.4 parts by weight per part by weight of the silica. More preferably, the amount of the compound of formula I will range from about 0.05 to about 0.25 parts by weight per part by weight of the silica.

In addition to the sulfur-containing organosilicon, the rubber composition should contain a sufficient amount of silica, and carbon black, if used, to contribute a reasonably high modulus and high resistance to tear. The silica filler may be added in amounts ranging from about 10 phr to about 250 phr. Preferably, the silica is present in an amount ranging from about 15 phr to about 80 phr. If carbon black is also present, the amount of carbon black, if used, may vary. Generally speaking, the amount of carbon black will vary from about 5 phr to about 80 phr. Preferably, the amount of carbon black will range from about 10 phr to about 40 phr. It is to be appreciated that the silica coupler may be used in conjunction with a carbon black, namely pre-mixed with a carbon black prior to addition to the rubber composition, and such carbon black is to be included in the aforesaid amount of carbon black for the rubber composition formulation. In any case, the total quantity of silica and carbon black will be at least about 30 phr. The combined weight of the silica and carbon black, as hereinbefore referenced, may be as low as about 30 phr, but is preferably from about 45 to about 130 phr.

The commonly employed siliceous pigments used in rubber compounding applications can be used as the silica in this invention, including pyrogenic and precipitated siliceous pigments (silica), although precipitate silicas are preferred. The siliceous pigments preferably employed in this invention are precipitated silicas such as, for example, those obtained by the acidification of a soluble silicate, e.g., sodium silicate.

Such silicas might be characterized, for example, by having a BET surface area, as measured using nitrogen gas, preferably in the range of about 40 to about 600, and more usually in a range of about 50 to about 300 square meters per gram. The BET method of measuring surface area is described in the Journal of the American Chemical Society, Volume 60, page 304 (1930).

The silica may also be typically characterized by having a dibutylphthalate (DBP) absorption value in a range of about 100 to about 400, and more usually about 150 to about 300. The silica might be expected to have an average ultimate particle size, for example, in the range of 0.01 to 0.05 micron as determined by the electron microscope, although the silica particles may be even smaller, or possibly larger, in size.

Various commercially available silicas may be considered for use in this invention such as, only for example herein, and without limitation, silicas commercially available from PPG Industries under the Hi-Sil trademark with designations 210, 243, etc; silicas available from Rhone-Poulenc, with, for example, designations of Z1165MP and Z165GR and silicas available from Degussa AG with, for example, designations VN2 and VN3.

Tire tread formulations which include silica and an organosilicon compound will typically be mixed utilizing a thermomechanical mixing technique. The mixing of the tire tread rubber formulation can be accomplished by methods known to those having skill in the rubber mixing art. For example, the ingredients are typically mixed in at least two stages, namely at least one non-productive stage followed by a productive mix stage. The final curatives including sulfur-vulcanizing agents are typically mixed in the final stage which is conventionally called the "productive" mix stage in which the mixing typically occurs at a temperature, or ultimate temperature, lower than the mix temperature(s) than the preceding non-productive mix stage(s). The rubber, silica and sulfur-containing organosilicon, and carbon black if used, are mixed in one or more non-productive mix stages. The terms "non-productive" and "productive" mix stages are well known to those having skill in the rubber mixing art. The sulfur-vulcanizable rubber composition containing the sulfur-containing organosilicon compound, vulcanizable rubber and generally at least part of the silica should be subjected to a thermomechanical mixing step. The thermomechanical mixing step generally comprises a mechanical working in a mixer or extruder for a period of time suitable in order to produce a rubber temperature between 140° C. and 190° C. The appropriate duration of the thermomechanical working varies as a function of the operating conditions and the volume and nature of the components. For example, the thermomechanical working may be for a duration of time which is within the range of about 2 minutes to about 20 minutes. It will normally be preferred for the rubber to reach a temperature which is within the range of about 145° C. to about 180° C. and to be maintained at said temperature for a period of time which is within the range of about 4 minutes to about 12 minutes. It will normally be more preferred for the rubber to reach a temperature which is within the range of about 155° C. to about 170° C. and to be maintained at said temperature for a period of time which is within the range of about 5 minutes to about 10 minutes.

The polybutadiene rubber blend containing tire tread compounds of this invention can be used in tire treads in conjunction with ordinary tire manufacturing techniques. Tires are built utilizing standard procedures with the polybutadiene rubber blend simply being substituted for the rubber compounds typically used as the tread rubber. After the tire has been built with the polybutadiene rubber-containing blend, it can be vulcanized using a normal tire cure cycle. Tires made in accordance with this invention can be cured over a wide temperature range. However, it is generally preferred for the tires of this invention to be cured at a temperature ranging from about 132° C. (270° F.) to about 166° C. (330° F.). It is more typical for the tires of this invention to be cured at a temperature ranging from about 143° C. (290° F.) to about 154° C. (310° F.). It is generally preferred for the cure cycle used to vulcanize the tires of this invention to have a duration of about 10 to about 20 minutes with a cure cycle of about 12 to about 18 minutes being most preferred.

By utilizing the polybutadiene rubber blends of this invention in tire tread compounds, traction characteristics can be improved without compromising tread wear or rolling resistance. Since the polybutadiene rubber blends of this invention do not contain styrene, the cost of raw materials can also be reduced. This is because styrene and other vinyl aromatic monomers are expensive relative to the cost of 1,3-butadiene.

This invention is illustrated by the following examples which are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, all parts and percentages are given by weight.

EXAMPLE 1

In this example, an isoprene rubber having a high Tg (glass transition temperature) and a low molecular weight was synthesized by the technique of this invention using a mixed tertiary amine and sodium-t-amylate (STA) as the modifier system. In the procedure used, 2,000 grams of a silica/molecular sieve/aluminum dried premix containing 17.8 weight percent of isoprene in hexanes was charged into a one-gallon (3.8 liters) reactor. After the impurity of the premix was removed by an n-butyl lithium (n-BuLi) solution, 3.56 ml (milliliters) of a 2.0M solution of TMEDA (N,N,N',N'-tetramethylethylenediamine in hexane), 3.56 ml of a 2.0M solution of STA (in hexane) and 3.60 ml of a 0.96M solution of n-BuLi (in hexane) were added to the reactor. The molar ratio of STA to TMEDA and to n-BuLi was 2:2:1. The target Mn (number averaged molecular weight) was 100,000. The polymerization was allowed to proceed at 40° C. for an hour. An analysis of the residual monomer indicated that isoprene was all consumed. Then, 2.0 ml of a 2M solution of ethanol (in hexane) was added to the reactor to shortstop the polymerization and the polymer was removed from the reactor and stabilized with 1 phr (parts per 100 parts by weight of rubber) of antioxidant.

After the hexane solvent was evaporated, the resulting isoprene rubber was dried in a vacuum oven at 50° C. The isoprene rubber was determined to have a Tg at +14° C. It was also determined to have a microstructure which contained 15 percent 1,2-polyisoprene units, 82 percent 3,4-polyisoprene units and 3 percent 1,4-polyisoprene units. The Mooney viscosity (ML 1+4) at 100° C. of the isoprene rubber made was determined to be 13. The molecular weight of this isoprene rubber was measured using GPC (gel permeation chromatography) to have an Mn (number averaged molecular weight) of 78,000 and an Mw (weight averaged molecular weight) of 99,000 and a molecular weight distribution (MWD) of 1.30.

EXAMPLE 2

The procedure described in Example 1 was utilized in this example except that the target number average molecular weight (M_(n)) of the isoprene rubber was increased from 100,000 to 200,000. To achieve this, the amount of the modifiers and n-BuLi was reduced to half; i.e., 1.78 ml of a 2M solution of STA, 1.78 ml of a 2M solution of TMEDA and 1.8 ml of a 0.96M solution of n-BuLi were used in this experiment. The isoprene rubber formed was determined to have a Tg at +18° C. It was also determined to have a microstructure which contained 15 percent 1,2-polyisoprene units, 79 percent 3,4-polyisoprene units and 6 percent 1,4-polyisoprene units. The Mooney viscosity (ML 1+4) at 100° C. of the isoprene rubber made was determined to be 37. The molecular weight of this isoprene rubber was measured using GPC to have a M_(n) of 121,000 and a M_(w) (weight averaged molecular weight) of 182,000 and a molecular weight distribution (MWD) of 1.50.

EXAMPLE 3

The procedure described in Example 1 was utilized in this example except that the STA to TMEDA to n-BuLi ratio was changed from 2:2:1 to 0.5:0.5:1. The isoprene rubber formed was determined to have a Tg at +100° C. It was also determined to have a microstructure which contained 12 percent 1,2-polyisoprene units, 73 percent 3,4-polyisoprene units and 12 percent 1,4-polyisoprene units. The Mooney viscosity (ML 1+4) at 100° C. of the isoprene rubber made was determined to be 9. The molecular weight of this isoprene rubber was measured using GPC (gel permeation chromatography) to have a M_(n) (number averaged molecular weight) of 72,000 and a M_(w) (weight averaged molecular weight) of 80,000 and a molecular weight distribution (MWD) of 1.12.

EXAMPLE 4

The procedure described in Example 1 was utilized in this example except that the STA to TMEDA to n-BuLi ratio was changed from 2:2:1 to 0.25:3:1 and the polymerization temperature was increased from 40° C. to 65° C. The GC (gas chromatography) analysis of the residual monomer indicated that 95 percent of isoprene monomer was consumed in 5 minutes. A complete monomer conversion was achieved after 20 minutes. The isoprene rubber produced was determined to have a Tg at +17° C. It was also determined to have a microstructure which contained 15 percent 1,2-polyisoprene units, 79 percent 3,4-polyisoprene units and 6 percent 1,4-polyisoprene units. The Mooney viscosity (ML 1+4) at 100° C. of the polyisoprene rubber made was determined to be 19. This polyisoprene rubber was measured to have a Mn of 72,400 and a M_(w) of 86,000 and a molecular weight distribution (MWD) of 1.20.

EXAMPLE 5

The procedure described in Example 1 was utilized in this example except that the polymerization was changed from 40° C. to 30° C. and the STA to TMEDA to n-BuLi ratio was changed from 2:2:1 to 0.25:3:1. It took 20 minutes to convert 95 percent of isoprene monomer to polyisoprene rubber. The Tg, microstructure and Mooney viscosity of the polyisoprene rubber made are shown in Table 1.

Comparative Example 6

The procedure described in Example 1 was utilized in this example except that the polymerization was changed from 40° C. to 30° C. and the modifier was changed from mixed TMEDA/STA to TMEDA alone. It took 280 minutes to convert 95 percent of isoprene monomer to polyisoprene rubber. The Tg, microstructure and Mooney viscosity of the polyisoprene rubber made are shown in Table 1.

Comparative Example 7

The procedure described in Example 1 was utilized in this example except that the polymerization temperature was decreased from 40° C. to 30° C. and except that the modifier was changed from a mixed TMEDA/STA to diglyme (2-methoxyethyl ether). It took 180 minutes to convert 95 percent of the isoprene monomer to polyisoprene rubber. The Tg and microstructure of the polyisoprene rubber made are shown in Table 1.

Comparative Example 8

The procedure described in Example 1 was utilized in this example except that the polymerization temperature employed was changed from 40° C. to 30° C. and the modifier was changed from mixed TMEDA/STA to ETE (ethyltetrahydrofurfuryl ether). It took 140 minutes to convert 95 percent of the isoprene monomer to polyisoprene rubber. The Tg, microstructure and Mooney ML-4 viscosity of the polyisoprene rubber made are shown in Table 1.

                                      TABLE 1     __________________________________________________________________________                Time taken for                           ML  Microstructures     Ex. No.         Modifier                95% conv.                       Tg  1 + 4.sup.1                               1,2-PI                                   1,4-PI                                       3,4-PI     __________________________________________________________________________     4   STA/TMEDA                 5 min.                       +17° C.                           19  15% 6%  79%     5   STA/TMEDA                 20 min.                       +19° C.                           22  14% 2%  84%     6   TMEDA  280 min.                        +8° C.                           12  15% 17% 68%     7   Diglyme                180 min.                       +12° C.                           15  12% 9%  79%     8   ETE    140 min.                       +13° C.                           18  14% 6%  80%     __________________________________________________________________________      .sup.1 The Mooney viscosity (ML 1 + 4) was measured at 100° C.

EXAMPLE 9

In this example, an isoprene rubber having an Mn of 300,000 was prepared by the technique of this invention using mixed tertiary amine and sodium-t-amylate (STA) as the modifier. In the procedure used, 2,250 grams of a silica/molecular sieve/aluminum dried premix containing 20 weight percent of isoprene in hexanes was charged into a one-gallon (3.8 liters) reactor. After the impurity of the premix was removed by a n-butyl lithium (n-BuLi) solution, 2.25 ml (milliliters) of a 2.0M solution of DPE (N,N'-1,2-diperidinuethane; in hexane), 1.5 ml of a 0.5M solution of STA (in hexane) and 1.56 ml of a 0.96M solution of n-BuLi (in hexane) were added to the reactor. The molar ratio of STA to DPE and to n-BuLi was 0.5:3:1. The polymerization was allowed to proceed at 50° C. An analysis of the residual monomer indicated that the polymerization was essentially complete (more than 95 percent of isoprene was consumed) after 30 minutes. The polymerization was continued for another 30 minutes for complete consumption of monomer. Then, 1.0 ml of a 2M solution of ethanol (in hexane) was added to the reactor to shortstop the polymerization and the polymer was removed from the reactor and stabilized with 1 phr (parts per 100 parts by weight of rubber) of antioxidant.

After the hexane solvent was evaporated, the resulting isoprene rubber was dried in a vacuum oven at 50° C. The isoprene rubber formed was determined to have a Tg at +30° C. It was also determined to have a microstructure which contained 89 percent 3,4-polyisoprene units, 9 percent 1,2-polyisoprene units and 2 percent 1,4-polyisoprene units.

EXAMPLE 10

The procedure described in Example 9 was utilized in this example except that 1,3-butadiene was used as the monomer in place of isoprene. It took 15 minutes to convert 95 percent of 1,3-butadiene monomer to butadiene rubber. The polybutadiene formed was determined to have a Tg at -90° C. It was also determined to have a microstructure which contained 93 percent 1,2-polybutadiene units and 7 percent 1,4-polybutadiene units.

Comparative Example 11

The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50° C. to 70° C. and the modifier was changed from STA/DPE to TMEDA alone. The Tg of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 2.

Comparative Example 12

The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50° C. to 70° C. and the modifier was changed from STA/DPE to ETE alone. The Tg of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 2.

EXAMPLE 13

The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50° C. to 70° C. and the modifiers was changed from STA/DPE to STA/TMEDA. The Tg of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 2.

EXAMPLE 14

The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50° C. to 70° C. and the modifiers were changed from STA/DPE to STA/ETE. The Tg of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 2.

                  TABLE 2     ______________________________________     Ex.   Modifier   PZN            Time needed to reach     No.   System     Temp     Tg    90+ % Conversion     ______________________________________     11    TMEDA      70° C.                               -16° C.                                     240 minutes     12    ETE        70° C.                               -15° C.                                     30 minutes     13    STA/ETE    70° C.                                -3° C.                                     20 minutes     14    STA/TMEDA  70° C.                                0° C.                                     15 minutes     ______________________________________

Comparative Example 15

The procedure described in Example 9 was utilized in this example except that 1,3-butadiene was used as monomer in place of isoprene and the modifier was changed from STA/DPE to TMEDA alone. The Tg of the polybutadiene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 3.

Comparative Example 16

The procedure described in Example 9 was utilized in this example except that 1,3-butadiene was used as monomer in place of isoprene and the modifier was changed from STA/DPE to ETE alone. The Tg of the polybutadiene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 3.

EXAMPLE 17

The procedure described in Example 9 was utilized in this example except that 1,3-butadiene was used as the monomer in place of isoprene and the modifiers were changed from STA/DPE to STA/TMEDA. The Tg of the polybutadiene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 3.

                  TABLE 3     ______________________________________     Ex.   Modifier   PZN            Time needed to reach     No.   System     Temp     Tg    90+ % Conversion     ______________________________________     15    TMEDA      50° C.                               -40° C.                                     60 minutes     16    ETE        50° C.                               -36° C.                                     45 minutes     17    STA/TMEDA  50° C.                               -22° C.                                     15 minutes     ______________________________________

Comparative Example 18

The procedure described in Example 9 was utilized in this example except that a premix containing 50:50 mixture of 1,3-butadiene and isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to TMEDA alone. The Tg of the 50/50 IBR (isoprene-butadiene rubber) formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 4.

Comparative Example 19

The procedure described in Example 9 was utilized in this example except that a premix containing 50:50 mixture of 1,3-butadiene and isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to ETE alone. The Tg of the 50/50 IBR formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 4.

EXAMPLE 20

The procedure described in Example 9 was utilized in this example except that a premix containing 50:50 mixture of 1,3-butadiene and isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to STA/ETE. The Tg of the 50/50 IBR formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 4.

EXAMPLE 21

The procedure described in Example 9 was utilized in this example except that a premix containing 50:50 mixture of 1,3-butadiene and isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to STA/TMEDA. The Tg of the 50/50 IBR formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 4.

                  TABLE 4     ______________________________________     Ex.   Modifier   PZN            Time needed to reach     No.   System     Temp     Tg    90+ % Conversion     ______________________________________     18    TMEDA      65° C.                               -26° C.                                     210 minutes     19    ETE        65° C.                               -27° C.                                     60 minutes     20    STA/ETE    65° C.                               -25° C.                                     20 minutes     21    STA/TMEDA  65° C.                               -16° C.                                     20 minutes     ______________________________________

Comparative Example 22

The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50° C. to 70° C. and the modifier was changed from STA/DPE to DMAMP (2-dimethyl aminomethyl-1-methyl piperidine) alone. The Tg, microstructure, Mooney viscosity (ML 1+4) of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 5.

Comparative Example 23

The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50° C. to 70° C. and the modifier was changed from STA/DPE to Sparteine alone. The Tg, microstructure, Mooney viscosity (ML 1+4) of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 5.

EXAMPLE 24

The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50° C. to 70° C. and the modifier was changed from STA/DPE to STA/DMAMP. The Tg microstructure, Mooney viscosity (ML 1+4) of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE 5.

EXAMPLE 25

The procedure described in Example 9 was utilized in this example except that the polymerization temperature was changed from 50° C. to 70° C. and the modifier was changed from STA/DPE to STA/Sparteine. The Tg, microstructure, Mooney viscosity (ML 1+4) of the polyisoprene formed and the time needed to consume more than 90 percent of the monomer charged are listed in TABLE

                                      TABLE 5     __________________________________________________________________________     Ex.        Modifier               PZN     ML  Microstructure                                       Time needed for     No.        System Temp                   Tg  1 + 4.sup.1                           1,2-PI                               1,4-PI                                   3,4-PI                                       90+ % Conversion     __________________________________________________________________________     22 DMAMP  70° C.                   -32° C.                       43  2   54  44  120 minutes     23 Sparteine               70° C.                   -60° C.                       45  0   87  13  180 minutes     24 STA/DMAMP               70° C.                    -2° C.                       56  9   24  67   30 minutes     25 STA/Spartein               70° C.                   -12° C.                       43  5   36  59   45 minutes     __________________________________________________________________________      .sup.1 The Mooney viscosity (ML 1 + 4) was measured at 100° C.

EXAMPLE 26

The procedure described in Example 9 was utilized in this example except that a premix containing 10:90 mixture of styrene and 1,3-butadiene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to STA/TMEDA. It took 10 minutes to consume more than 90 percent of the monomers charged. The 10/90 SBR (styrene-butadiene rubber) produced was determine to have a Tg at -20° C. It was also determined to have a Mooney viscosity (ML 1+4) at 100° C. of 69.

EXAMPLE 27-31

The procedure described in Example 9 was utilized in this example except that isoprene monomer was replaced with 25:75, 30:70, 35:65, 40:60 and 45:55 mixtures of styrene and 1,3-butadiene in hexanes and the modifier was changed from STA/DPE to STA/TMEDA. The STA to TMEDA and to n-BuLi ratio was also changed to 0.5:0.3:1. It took about 15 minutes to consume more than 90 percent of the monomers charged at the polymerization temperature of 70° C. for all these experiments. Based on proton NMR (nuclear magnetic resonance) measurements, the polystyrenes contained in these SBRs are randomly distributed in the polymer chains. The Tgs and microstructures of these SBRs made are listed in TABLE 6.

                                      TABLE 6     __________________________________________________________________________                                Microstructure         Modifier                Styrene:Butadiene                        PZN             Styrene     Ex. No.         System Ratio   Temp                            Tg  1,2-PBd                                    1,4-PBd                                        (Random)     __________________________________________________________________________     27  STA/TMEDA                25:75   70° C.                            -35° C.                                39  36  25     28  STA/TMEDA                30:70   70° C.                            -30° C.                                38  33  29     29  STA/TMEDA                35:65   70° C.                            -27° C.                                32  34  34     30  STA/TMEDA                40:60   70° C.                            -21° C.                                30  30  40     31  STA/TMEDA                45:55   70° C.                            -13° C.                                28  28  44     __________________________________________________________________________

EXAMPLE 32-32

The procedure described in Example 9 was utilized in this example except that isoprene monomer was replaced with 40:60 and 45:55 mixtures of styrene and 1,3-butadiene in hexanes and the modifier was changed from STA/DPE to STA/TMEDA. The STA to TMEDA and to n-BuLi ratio was also changed to 0.5:0.5:1. It took about 15 minutes to consume more than 90 percent of the monomers charged at the polymerization temperature of 70° C. for all these experiments. Based on proton NMR (nuclear magnetic resonance) measurements, the polystyrenes contained in these SBRs are randomly distributed in the polymer chains. The Tgs and microstructures of these SBRs made are listed in TABLE is 7.

                                      TABLE 7     __________________________________________________________________________                                Microstructure         Modifier                Styrene:Butadiene                        PZN             Styrene     Ex. No.         System Ratio   Temp                            Tg  1,2-PBd                                    1,4-PBd                                        (Random)     __________________________________________________________________________     32  STA/TMEDA                40:60   70° C.                            -12° C.                                35  26  39     33  STA/TMEDA                45:55   70° C.                             -8° C.                                30  25  45     __________________________________________________________________________

Comparative Example 34

The procedure described in Example 9 was utilized in this example except that a premix containing 25:75 mixture of styrene and 1,3-isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to ETE alone. The molar ratio of ETE to n-BuLi was 0.5:1. The Tg, microstructure of 25/75 SIR (styrene-isoprene rubber) formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 8.

EXAMPLE 35

The procedure described in Example 9 was utilized in this example except that a premix containing 25:75 mixture of styrene and 1,3-isoprene in hexanes was used in place of isoprene and the modifier was changed from STA/DPE to STA/ETE. The molar ratio of STA to ETE and to n-BuLi was 0.5:0.5:1. The Tg, microstructure of 25/75 SIR formed and the time needed to consume more than 90 percent of the monomers charged are listed in TABLE 8.

                                      TABLE 8     __________________________________________________________________________     Ex.        Modifier             PZN     Microstructure  Time needed for     No.        System             Temp                 Tg  1,2-PI                         1,4-PI                             3,4-PI                                 Styrene                                     90+ % Conversion     __________________________________________________________________________     34 ETE  70° C.                 -30° C.                     2   52  21  25  30 minutes     35 STA/ETE             70° C.                 -10° C.                     5   27  52  25  20 minutes     __________________________________________________________________________

EXAMPLE 36-37

The procedure described in Example 9 was utilized in this example except that isoprene monomer was replaced with 10:90 mixture of styrene and isoprene in hexanes and the modifier was changed from STA/DPE to STA/TMEDA or STA/ETE. It took about 10 minutes to consume more than 90 percent of the monomers charged at the polymerization temperature of 70° C. for all these experiments. Based on proton NMR measurements, the polystyrenes contained in these SIRs are randomly distributed in the polymer chains. The Tgs and microstructures of these SIRs made are listed in TABLE 9.

                                      TABLE 9     __________________________________________________________________________                                Microstructure         Modifier                Styrene:Butadiene                        PZN                 Styrene     Ex. No.         System Ratio   Temp                            Tg  1,2-PI                                    1,4-PI                                        3,4-PI                                            (Random)     __________________________________________________________________________     36  STA/TMEDA                10:90   70° C.                            +14° C.                                12  10  68  10     37  STA/ETE                10:90   70° C.                             +3° C.                                11  18  61  10     __________________________________________________________________________

EXAMPLE 38

The procedure described in Example 9 was utilized in this example except that isoprene monomer was replaced with 25:50:25 mixture of styrene, isoprene and 1,3-butadiene in hexanes and the modifier was changed from STA/DPE to STA/ETE. It took about 12 minutes to consume more than 90 percent of the monomers charged at the polymerization temperature of 70° C. Based on proton NMR measurements, the polystyrenes contained in the SIBR (styrene-isoprene-butadiene rubber) are randomly distributed in the polymer chains. The SIBR made was determined to have a Tg at +1° C. It was also determined to have a microstructure which contained 24 percent polystyrene units, 13 percent 1,2-polybutadiene units, 9 percent 1,4-polybutadiene units, 7 percent 1,2-polyisoprene units, 11 percent 1,4-polyisoprene units and 36 percent 3,4-polyisoprene units. The SIBR was also measured to have a Mooney viscosity (ML 1+4) at 100° C. of 75.

Comparative Example 39

In this experiment, a blend was made by mixing 40 parts of natural rubber, 15 parts of high-cis-1,4-polybutadiene, 30 parts of solution styrene-butadiene rubber, 15 parts of a conventional low Tg 3,4-polyisoprene rubber, 38 parts of carbon black, 10 parts of Hi-Sil™ 210 silica, 2 parts of Degussa X50S coupling agent, 10 parts of a paraffinic process oil, 3.5 parts of antidegradants, 3.5 parts of zinc oxide, 2 parts of stearic acid, 1 part of N-cyclohexyl-benzenethiazole-2-sulfenamide, 0.1 part of tetramethylthiuram disulfide and 1.5 parts of sulfur. The low Tg 3,4-polyisoprene rubber utilized in this blend had a Tg of -16° C. and a number average molecular weight of about 350,000. This rubber blend was subsequently cured and evaluated for physical properties which are reported in TABLE 10.

Low tan delta values at 70° C. are indicative of low hysteresis and consequently tire treads made with such rubbers exhibit lower rolling resistance than tires made with rubbers having higher tan delta values at 70° C. On the other hand, rubbers which have high tan delta values between -25° C. and +25° C. can be utilized in making tire treads which exhibit better traction characteristics than tires made with tread rubber compositions which have lower tan delta values over the -25° C. to +25° C. temperature range. It is preferred for a tread rubber composition to have the highest possible tan delta value at every temperature within the range of -25° C. to +25° C. to realize the best possible traction characteristics over all driving conditions and at all driving speeds.

The rubber blend made in this experiment had a high tan delta value of greater than about 0.25 at temperatures of less than about -15° C. However, the tan delta values of this blend dropped rapidly with increasing temperatures above -10° C. At 0° C., this blend exhibited a tan delta value of about 0.18.

This experiment shows that conventional low Tg 3,4-polyisoprene rubber can be used in making tire tread formulations which have high tan delta values at temperatures of less than about 0° C. The low tan delta value at 70° C. of less than 0.1 further shows that this rubber formulation can be used to make tire treads which exhibit good rolling resistance. However, it would be highly desirable for the tan delta value of this rubber blend to be higher at temperatures within the range of about 0° C. to about 25° C. to exhibit outstanding traction characteristics over all driving conditions and at all driving speeds.

EXAMPLE 40

In this experiment, a tire tread rubber formulation was made using the same procedure as was employed in Example 39 except for the fact that a high Tg 3,4-polyisoprene rubber was substituted for the low Tg 3,4-polyisoprene rubber utilized in Example 39. The high Tg 3,4-polyisoprene rubber employed in this experiment had a M_(n) of 121,000, a M_(w) of 182,000 and a M_(z) of 245,000. It was synthesized by the procedure described in Example 2.

The rubber formulation made in this experiment had the physical properties reported in TABLE 10. As can be seen, the tan delta values of the blend increased over the temperature range from about -2° C. to about +23° C. This blend has a tan delta value at 70° C. of about 0.07. This low tan delta value at 70° C. of less than 0.1 shows that this rubber formulation can be used to make tire treads which exhibit good rolling resistance. This unique combination of high tan delta values at low temperatures and low tan delta values at high temperatures is indicative of a tire tread formulation with both good traction characteristics and good rolling resistance.

Generally, as the glass transition temperature of a tire tread rubber formulation increases, abrasion resistance is sacrificed. However, it has been unexpectedly found that this is not the case with the tire tread rubber formulations of this invention. In fact, it has been determined that the tire tread rubber formulations of this invention have DIN abrasion characteristics which are as good as or better than the lower Tg control compounds. This means that better tread life can be expected while also attaining improved traction characteristics.

EXAMPLE 41

In this experiment, a tire tread rubber formulation was made using the same procedure as was employed in Example 39 except for the fact that a high Tg 3,4-polyisoprene rubber was substituted for the low Tg 3,4-polyisoprene rubber utilized in Example 39. The high Tg 3,4-polyisoprene rubber employed in this experiment had a M_(n) of 72,000 and a M_(w) of 86,000.

The rubber formulation made in this experiment had the physical properties reported in TABLE 10. As can be seen, the tan delta values of the blend increased over the temperature range from about -5° C. to about +17° C. This blend had a tan delta value at 70° C. of about 0.08. This low tan delta valued at 70° C. of less than 0.1 shows that this rubber formulation can be used to make tire treads which exhibit good rolling resistance. This unique combination of high tan delta values at low temperatures and low tan delta values at high temperatures is indicative of a tire tread formulation with both good traction characteristics and good rolling resistance.

Generally, as the glass transition temperature of a tire tread rubber formulation increases, abrasion resistance is sacrificed. However, it has been unexpectedly found that this is not the case with the tire tread rubber formulations of this invention. In fact, it has been determined that the tire tread rubber formulations of this invention have DIN abrasion characteristics which are as good as or better than the lower Tg control compounds. This means that better tread life can be expected while also attaining improved traction characteristics.

                  TABLE 10     ______________________________________     Example     39         40         41     ______________________________________     100% Modulus                 1.9 MPa    2.0 MPa    2.0 MPa     300% Modulus                 8.5 MPa    9.0 MPa    8.7 MPa     Break Strength                 16.2 MPa   14.6 MPa   14.3 MPa     Elongation.sup.1                 515%       469%       469%     Hardness (20° C.)                 58         60         60     Hardness (100° C.).                 52         52         51     Rebound (20° C.)                 49         40         39     Rebound (100° C.)                 67         66         65     DIN Abrasion.sup.2                 116        105        111     tan delta @ -25° C.                 0.28       0.20       0.21     tan delta @ -20° C.                 0.26       0.17       0.18     tan delta @ -15° C.                 0.25       0.15       0.16     tan delta @ -10° C.                 0.24       0.14       0.15     tan delta @ -5° C.                 0.22       0.14       0.15     tan delta @ 0° C.                 0.18       0.14       0.15     tan delta @ 5° C.                 0.16       0.14       0.17     tan delta @ 10° C.                 0.13       0.16       0.21     tan delta @ 15° C.                 0.12       0.19       0.23     tan delta @ 20° C.                 0.11       0.23       0.23     tan delta @ 25° C.                 0.10       0.23       0.19     tan delta @ 70° C.                 0.07       0.07       0.08     ______________________________________      .sup.1 Elongation was measured to break.      .sup.2 DIN Abrasion was measured as volume lost in cubic centimeters      (lower is better).

EXAMPLE 42

In this experiment, 1500 g of a silica/alumina/molecular sieve dried premix containing 12.4 weight percent 1,3-butadiene in hexane was charged into a one-gallon (3.8 liters) reactor. After the impurity level of 1.5 ppm was determined, 2.76 ml of 1M solution of N,N,N',N'-tetramethylethylene diamine (TMEDA) in hexane, 1.84 ml 0.5M solution of sodium-t-amylate (STA) in hexane and 1.15 ml of a 0.91M solution of n-butyllithium (in hexane; 1.01 ml for initiation and 0.14 ml for scavenging the premix) were added to the reactor. The molar ratio of TMEDA to STA and to n-butyllithium was 3:1:1.

The polymerization was carried out at 75° C. for 10 minutes. The GC analysis of the residual monomer contained in the polymerization mixture indicated that the polymerization was complete at this time. Then, 3 ml of 1M ethanol solution (in hexane) was added to the reactor to shortstop the polymerization and polymer was removed from the reactor and stabilized with 1 phm of antioxidant. After evaporating hexane, the resulting polymer was dried in a vacuum oven at 50° C.

The polybutadiene produced was determined to have a glass transition temperature (Tg) at -27° C. It was then determined to have a microstructure which contained 82 percent 1,2-polybutadiene units and 18 percent 1,4-polybutadiene units. The Mooney ML-4 viscosity ML-4 at 100° C. was 54 for this polybutadiene.

EXAMPLES 43-47

The procedure described in Example 42 was utilized in these examples except that the TMEDA/STA/n-butyllithium ratio was varied. The Tgs and microstructures of the resulting polybutadienes are listed in Table 11.

                  TABLE 11     ______________________________________     TMEDA/STA/n-BuLi                     Tg      Mooney  Microstructure     Ex.   Ratio         (° C.)                                 ML-4  1,2-PBd                                              1,4-PBd     ______________________________________     42    3:1:1         -28     54    82     18     43    3:2:1         -27     65    83     17     44    3:3:1         -27     68    82     18     45    3:0.5:1       -28     50    82     18     46    2:1:1         -27     50    82     18     47    5:1:1         -27     57    83     17     ______________________________________

EXAMPLE 48

The procedure described in Example 42 was utilized in this example except that TMEDA was used as the modifier. It took about 60 minutes to complete the polymerization. The polymer was determined to have a Tg at -45° C. It was also determined to have a microstructure which contained 63 percent 1,2-polybutadiene units and 37 percent 1,4-polybutadiene units.

EXAMPLE 49

The procedure described in Example 42 was utilized in this example except that TMEDA was used as the modifier and the TMEDA to n-butyl-lithium ratio was changed to 1:1. It took about 75 minutes to complete the polymerization. The polymer was determined to have a Tg at -63° C. It was also determined to have a microstructure which contained 50 percent 1,2-polybutadiene units and 50 percent 1,4-polybutadiene units.

EXAMPLES 50-55

The super-high vinyl polybutadiene rubber made in Example 42, the high vinyl polybutadiene rubber made in Example 48 and the medium vinyl polybutadiene rubber made in Example 49 were blended together and compounded utilizing a standard tire tread test formulation and compared to tire tread formulations made with a blend of solution styrene-butadiene rubber and high cis-1,4-polybutadiene rubber. The tire tread test formulations were made by mixing the ingredients shown in Table 12. Example 50 was carried out as a comparative example and with the rubber component being a blend of solution styrene-butadiene rubber and high cis-1,4-polybutadiene rubber.

                  TABLE 12     ______________________________________     Example    50     51      52    53   54    55     ______________________________________     SBR.sup.1  70     --      --    --   --    --     Cis-1,4-PBD.sup.2                30     --      --    --   --    --     MVPBD.sup.3                --     50      30    50   30    20     HVPBD.sup.4                --     30      50    30   50    70     SHVPBD.sup.5                --     20      20    20   20    10     carbon black                70     70      70    --   --    --     Silica     --     --      --    70   70    70     Coupling Agent.sup.6                --     --      --    11   11    11     wax         2      2       2     2    2     2     Oil        28     28      28    28   28    28     zinc oxide 3.5    3.5     3.5   3.5  3.5   3.5     stearic acid                 2      2       2     2    2     2     6-PPD.sup.7                 1      1       1     1    1     1     CBS.sup.8  1.7    1.7     1.7   1.7  1.7   1.7     DPG.sup.9   2      2       2     2    2     2     sulfur     1.4    1.4     1.4   1.4  1.4   1.4     Antioxidant                 1      1       1     1    1     1     ______________________________________      .sup.1 The solution styrenebutadiene rubber contained 10% bound styrene.      .sup.2 The high cis1,4-polybutadiene rubber was Budene ® 1207      polybutadiene rubber.      .sup.3 MVPBD represents medium vinyl polybutadiene rubber having a vinyl      content of about 50 percent.      .sup.4 HVPBD represents high vinyl polybutadiene rubber having a vinyl      content of about 63 percent.      .sup.5 SHVPBD represents superhigh vinyl polybutadiene rubber having a      vinyl content of about 82 percent.      .sup.6 The coupling agent was 50 percent active Si69 which was provided b      Degussa as X50S.      .sup.7 6PPD represents N(1,3-dimethyl butyl)Nphenyl-p-phenylenediamine.      .sup.8 CBS represents Ncyclohexyl-2-benzothiazyl-sulfenamide.      .sup.9 DPG represents diphenyl guanidine.      The physical properties of the compounded tire tread formulations are      reported in Table 13.

                  TABLE 13     ______________________________________     Compound Physical Properties     Example      50     51     52    53   54   55     ______________________________________     100% Modulus, MPa                   1.8    1.7    1.7   2.1  2.3  2.2     300% Modulus, MPa                   7.3    7.1    7.1   9.8  9.8  9.4     Break Strength, MPa                  15.5   15.6   15.3  15.3 15.7 14.7     Elongation at Break                  580%   587%   577%  438% 459% 443%     Hardness @ 23° C.                  65.7   64.5   64.1  61.6 65.5 64.3     Hardness @ 100° C.                  54.5   53.2   51.5  55.6 58.7 58.2     Rebound @ 23° C.                  34.3   31.4   30.0  41.6 38.3 38.8     Rebound @ 100° C.                  50.4   50.7   49.7  64.5 64.0 63.6     ______________________________________

The high rebound at 100° C. reported in Examples 53-55 is indicative of improved rolling resistance in tire tread formulations. The lower rebound at room temperature suggests better traction characteristics. Thus, the tire tread compounds made with silica showed an exceptional combination of characteristics for utilization in tire treads.

Variations in the present invention are possible in light of the description of it provided herein. It is, therefore, to be understood that changes can be made in the particular embodiments described which will be within the full intended scope of the invention as defined by the following appended claims. 

What is claimed is:
 1. An initiator system which is comprised of (a) a lithium initiator, (b) a sodium alkoxide and (c) a polar modifier; wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.15:1 to about 1:1; wherein the molar ratio of the polar modifier to the lithium initiator is within the range of about 0.4:1 to about 3:1; wherein the sodium alkoxide is of the formula NaOR, wherein R represents an alkyl group containing from about 3 to about 8 carbon atoms; and wherein said polar modifier is selected from the group consisting of diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine, triethylamine, N,N,N',N'-tetramethylethylenediamine, N-methyl morpholine, N-ethyl morpholine, N-phenyl morpholine and alkyltetrahydrofurfuryl ethers.
 2. An initiator system as specified in claim 1 wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.3:1 to about 0.5:1; and wherein the molar ratio of the polar modifier to the lithium initiator is within the range of about 0.5:1 to about 2:1.
 3. An initiator system as specified in claim 2 wherein the sodium alkoxide is of the formula NaOR, wherein R represents an alkyl group containing from about 4 to about 6 carbon atoms.
 4. An initiator system as specified in claim 3 wherein the lithium initiator is selected from the group consisting of ethyllithium, isopropyllithium, n-butyllithium, secondary-butyllithium, normal-hexyllithium, tertiary-octyllithium, phenyllithium, 2-napthyllithium, 4-butylphenyllithium, 4-phenylbutyllithium and cyclohexyllithium.
 5. An initiator system as specified in claim 4 wherein the sodium alkoxide is sodium t-pentoxide.
 6. An initiator system as specified in claim 4 wherein said polar modifier is N,N,N',N'-tetramethyl ethylenediamine.
 7. An initiator system as specified in claim 3 wherein the lithium initiator is n-butyllithium. 